Pigmented ink composition

ABSTRACT

Re-dispersible pigmented printing ink compositions, preferably for the continuous ink jet printing process are obtained by the addition of a flocculating agent to the pigment dispersion in the printing liquid. The compositions do not form hard sediments and are easy to re-disperse on stopping and resuming the printing operations. Suitable flocculating agents bind the pigment particles in the sediment into three-dimensional “airy” structures, which are easy to break up. The ink compositions of the invention are furthermore characterized by a good printability in the dispersed state.

FIELD OF INVENTION

The invention is in the field of inks preferably inks for thecontinuous-ink-jet printing. It concerns in particular pigmentedsecurity ink-jet ink formulations, containing inorganic luminescentpigments, which are easy to put into homogeneous dispersion condition(i.e. which have a good redispersibility) upon startup of the printingequipment, which remain homogeneously dispersed during printingoperation, which avoid obstruction (clogging) of the printer nozzles,and which furthermore result in printed features of excellent contrast,adherence, as well as resistance against light, humidity, bleach andother adverse environmental influences.

BACKGROUND OF THE INVENTION

In the technique of continuous-ink-jet printing, such as first describedin U.S. Pat. No. 3,369,252, a stream of liquid ink is subdivided into asequence of small droplets by a piezoelectric resonator, vibrating at afrequency of the order of 50 kHz or more. The droplets are projected outof a nozzle towards the substrate. The electric potential against groundof said nozzle, and thus the electric charge of the resulting inkdroplets, is controlled by an addressing electrode. Two deflectorelectrodes, arranged along the projection path of said ink droplets, areheld on predefined, constant electric potentials. They deflect the inkdroplets according to their electric charge and make them impact ondifferent locations on the substrate. By applying a correspondingelectric pulse of appropriate voltage and duration to the addressingelectrode, a single ink droplet can be addressed and directed to aspecific location on the substrate. In the absence of an addressingvoltage, the projected ink droplets are collected by a small collectingfunnel, from where they are recycled to an ink reservoir. Refinements tothe described printing method and equipment include the use of unchargedor slightly opposite charged guard drops between the printing droplets,such as first proposed in U.S. Pat. No. 3,562,757, in order to minimizethe effects of the mutual repulsion between droplets having the sameelectric charge. Further refinements to the printing method are proposedin U.S. Pat. No. 3,828,354; U.S. Pat. No. 3,833,910; U.S. Pat. No.3,846,800; U.S. Pat. No. 4,688,048; as well as in many otherpublications.

Inks for the continuous-ink-jet printing are normally of the soluble-dyetype, in order to provide for the necessary homogeneity, and thus, aneasy runnability on the printing machine. The inks may furthermore beformulated as solvent-based or water-based liquids. The later option isincreasingly preferred, due to the ever growing environmentalrestrictions which apply to the use of organic solvents. However,printings made with such inks lack in general resistance againstenvironmental influence, and they are also affected by solvents and/orwater. Waterfast inks in particular are highly desirable for theproduction of permanent indicia or images.

Pigmented inks for the continuous-ink-jet printing are known in the artand first described in U.S. Pat. No. 4,365,035. More recent publicationsinclude EP 751,194 and EP 853,106. Particular technical problems arisefrom the heterogeneous nature of the ink, which must be solved throughthe use of particular dispersing additives, and, principally, through acontinuous stirring and circulating of the ink during printingoperation.

Pigment particles for ink-jet printing are preferably smaller than 1micrometer, more preferably even smaller than 300 nanometers. However,the presence of some particles as large as 5 micrometers can betolerated in an ink-jet formulation. In particular, ink-jet inks aregenerally passed through a filter of 10 micrometer pore size, in orderto eliminate pigment fractions or agglomerates larger than this, whichcould obstruct the printing nozzles.

Existing solutions to maintain the pigmented ink dispersed duringprinting operation, and to prevent hard settling of the pigment betweenprinting operations, include the following approaches:

-   -   In U.S. Pat. No. 6,062,682 the ink cartridge is moved along a        carrier shaft such as to stir the pigment, and in U.S. Pat. No.        5,451,987 the ink- and recovery-reservoirs are equipped with        means, such as a magnetic stirrer, for agitating the ink.    -   U.S. Pat. No. 5,650,802 suggests the use of a vibrating piezo        element to prevent the pigment suspension from settling outside        the printing periods.    -   Alternatively, a re-circulation of the ink in the system at        regular frequency has also been successfully used to maintain        the ink in an appropriate state of dispersion. Such a system is        currently exploited in the OPAQUE DOMINO CIJ printers.

In the field of security printing, it would be highly desirable to haveavailable ink-jet inks which resist to the influence of light and tocommonly available chemical agents, such as household bleach or solventsof the most various kind. Such inks could noteworthy serve for the rapidand reliable on-line numbering of security documents, such as banknotes,identity documents, lottery tickets, warranty labels and the like.

A particular type of waterfast security inks for the ink-jet numberingof lottery tickets has been disclosed in U.S. Pat. No. 5,498,283.However, the disclosed inks do merely contain an organic dye which hasimproved resistance to bleach; formulations containing inorganicluminescents are not disclosed.

To provide for an appropriate intensity of the luminescence response,inorganic security pigments have generally a particle size of greaterthan 300 nm. Example of such pigments can be found in the disclosures ofU.S. Pat. No. 5,891,361; U.S. Pat. No. 6,132,642 and WO 00/63317. Belowsaid particle size, the surface-to-volume ratio of the particles, andthus the influence of the surface defects responsible for luminescencequenching, becomes too important, resulting in a strong drop of theluminescence quantum yield.

The addition, to a continuous ink jet ink, of inorganic luminescents ofsaid mentioned types noteworthy presents a couple of particularproblems, which are due to the relatively high specific weight of saidinorganic compounds, as compared to that of the printing liquid, to therequired pigment size, as well as to the very low viscosity of theprinting fluid. This results in a strongly increased tendency of thepigment to sediment, forming furthermore relatively dense deposits whichare difficult to redisperse. After a stop and restart of the printingoperations, not all pigment is thus necessarily redispersed; some of itremaining settled in the corners of the ink reservoir. Moreover, hardsedimentation of the inorganic pigment, when occurring between printingoperations or during prolonged storage periods, is very difficult toredisperse and requires high shear mixing forces to put the pigment backinto suspension.

Hard sedimentation results in a change in concentration of the activeingredient with time, which is not compatible with the stringentstandards of security printing.

The combination of a particularly enhanced stirring and circulating ofthe printing liquid during operation, together with the use of an inkformulation according to the present invention, are efficiently used toremedy this problem.

SUMMARY OF THE INVENTION

The shortcomings of the inks according to the prior art are avoided byusing an ink composition of the present invention.

The inventors surprisingly found that a partial flocculation(destabilization) of the ink suspension turned out to be beneficial toits overall properties. The redispersion of a partially flocculated inksuspension turned out to be significantly improved, without perturbingthe print-ability of the ink.

The printing ink composition comprises at least one solvent, a bindersystem, optional defoaming, humectant and conductivity control agents,and at least one pigment; the composition being characterized in that itadditionally comprises at least one flocculating agent, whichflocculating agent is capable of forming a re-dispersible sediment ofsaid pigment in said composition.

The solvent is preferably selected from the group comprising water,alcohols (such as methanol, ethanol, etc.), ketones (such as acetone,methyl-ethyl-ketone, etc.) and esters (such as methyl acetate, ethylacetate, methyl propionate, etc.) or mixtures thereof.

The binder system comprises at least one polymer compound suitable forink-jet ink applications. Mixtures of different polymer compounds areusable as well. The polymer compounds are preferably chosen among thethermoplastic resins and are furthermore chosen such as to be compatiblewith the solvent. For aqueous ink formulations, the binder system ispreferably, but not exclusively, chosen from the group comprisingwater-soluble resins, such as vinylpyrrolidone polymers and copolymers,water soluble polyesters, salts of acrylic and maleic anhydride polymersand copolymers, polyvinyl alcohol, cellulose derivatives,polyethyleneimides, etc., or from the group of film forming emulsions offine polymer particles (<1 μm), e.g. latex polymers of the various kindsknown in the art.

For solvent especially organic solvent based systems, solvent solublethermoplastic resins are preferred, such as acrylic resins, e.g. poly(methylmethacrylate), vinylic resins, e.g. vinyl acetate/vinyl chloridecopolymers, polyvinylbutyral resins, cellulose resins, e.g.nitrocellulose, acetylcellulose etc., polyurethane resins, polyesterresins, or polyamine resins. Alternatively phenolic and epoxy resins canbe used as well.

Depending on the mole weight (M_(W)) of the binder resin, differentamounts of resin are incorporated into the inkjet ink formulation, inorder to reach a viscosity in the range of between 2 and 10 mPa.s (at25° C.), preferably of between 3 and 7 mPa.s (at 25° C.). Preferableamounts of binder lie between 0.5 and 20% and more preferably between 2%and 15% by weight of the ink's total weight.

The pigments used in an ink composition of the invention are preferablysecurity pigments, i.e. pigments which are not otherwise used incommercial printing processes, and may be selected either from organicor from inorganic pigments. Preferably, luminescent pigments are used.Organic luminescent pigments may include appropriate derivatives ofphthalocyanine, naphthalocyanine, perylene and other polycyclic aromatichydrocarbons, quinazolinone, oxazole, quinone, napthalimide, etc., ormixtures thereof. Preferably inorganic luminescent pigments areselected, chosen from the class of activator-doped crystallinesolid-state compounds, such as activator-doped zinc sulfides,oxysulfides, vanadates, fluorides, oxides and oxyfluorides of yttriumand the rare-earths, or mixtures thereof.

In order to stabilize or disperse the pigment in the ink compositionpreferably a dispersing agent is added to the ink composition. Thisdispersing agent allows to distribute the pigments homogeneouslythroughout the ink composition and further helps to ensure a continuousgood printability of the ink composition, even after a prolonged storageperiod.

By adding an appropriate amount of a flocculating agent to a pigmentedink-jet ink, a partial destabilisation of the printing fluid suspensionwas obtained, through a reversible, controlled, weak flocculation of thepigment. Said partial destabilisation resulted on one hand in a fastersettling of the pigment, but on the other hand yielded a much softersediment, which was very easily redispersed upon the application of asmall mechanical movement (stirring or shaking). In particular,sedimented inks could be completely redispersed using a simple magneticstirrer at 200 rpm for a duration of two minutes.

Flocculation is a technique known to the one skilled in the art, whichis used to render fine suspensions, being otherwise difficult to filteror to decant, coarser through the action of particle agglomeration. Inthe field of printing, in particular, two different types offlocculation processes are distinguished, i.e. a “controlled” or “slow”flocculation, resulting in a “soft” deposit, and an “uncontrolled” or“rapid” flocculation, resulting in a “hard” deposit. Their definitionsare given in Römpp's Lexikon “Lacke und Druckfarben” (p. 240, 1998), aswell as in Ullmanns “Encyclopädie der Technischen Chemie” (Vol. 11, p.581-586, 4^(th) edition, 1976) and the content of these definitions isincluded within the application. The “controlled flocculation” is thusdescribed as a slow sedimentation of the suspended particles, preventingthe creation of densly packed and difficult to redisperse sediments.“Controlled flocculating agents” are thus chemical compounds which, uponaddition to a suspension, allow to obtain a “soft sediment”. Theresulting flocculated product is generally redispersible, but itflocculates again upon removal of the agitation. This is due to the easeof formation and breaking of the chemical hydrogen bonds, coordinationbonds, or Van-der-Waals forces, which are associating the flocculatedparticles. A flocculating agent as used according to the invention isselected from these types of “controlled” flocculating agents which arecapable of forming redispersable “soft” sediments in such a way asdescribed above.

Provided that the amount of flocculating agent is properly dosed as afunction of pigment nature, amount and particle size, the extent offlocculation can be adjusted to fall between the two non-workableextremes, as found by our experimental work:

(i) A too strong flocculation (large amount of added flocculating agent)resulted in a very fast total sedimentation (<15 min.) but impartedstrongly on the stability of the printing fluid, resulting in a poorprint-ability of the ink.

(ii) A too weak flocculation (too small or zero amount of addedflocculating agent) resulted in a very slow sedimentation (>48 h); theresulting ink had a very good print-ability but was impossible toredisperse without applying high shear mixing forces after standing forsome days.

The selection of the flocculating agent used in a composition accordingto the invention is dependent on the specific properties of the solvent,the binder system or further required compounds of the composition, thusthe flocculating agent or its interactions with the components has to be“compatible” with these components of the ink composition; a personskilled in the art will be able to make the correct selection.

For water based liquids, flocculating agents (flocculents) can be of twotypes:

-   -   i) inorganic flocculating agents comprising inorganic salts,        especially salts of aluminum, calcium or iron(III) cations, for        example aluminum sulfate or iron(III) chloride, calcium        chloride, etc. These cations hydrolyze to form inorganic        hydroxy-polymers at the pH value of the solution. The fine        particles, suspended in the liquid containing the flocculent,        chemically bind to the hydrolyzed metal salt via the action of        hydrogen bridges; i.e. the particles get covered with a thin        layer of metal hydroxide. It is the interaction between the so        coated surfaces of the particles, which makes them associate,        forming larger aggregates which eventually precipitate.    -   ii) organic flocculating agents comprising organic polymers        carrying functional groups which are able to interact with and        bind to the surface of the particles. The flocculation mechanism        is the same as in the case of hydrolyzing metal salts. Examples        of such polymers are polyacrylates, hydrolyzed polyacrylamide,        polysulfonate, polyphosphates and polyphosphonates (anionic),        polyimines, quaternized polyamines (cationic), polyacrylamide        (nonionic), etc. Combinations of anionic and cationic        flocculents can also be employed to enhance the flocculation        effect.

For organic-solvent based liquids, the controlled flocculating agentsare principally but not exclusively of polymeric nature. They bind tothe particles and act by building “bridges” between single pigmentparticles through remaining free chemical functional groups, resultingin a three-dimensional network implying said particles. Examples of suchpolymers are polycarboxylic acids or salts of polycarboxylic acids,alkylammonium salts of polycarboxylic acids, polyamines or salts ofpolyamines, polycarboxylic acid salts of a polyamine amide, polyamides,alkylolammonium salts of an unsaturated fatty acid, low molecular weightunsaturated polycarboxylic acid polyesters, carboxylic acids orphosphonic acids having formula R₁PO₃(R₂)₂ (with R₁═H, alkyl; R₂═H,alkyl or aryl) and their salts, etc. or mixtures thereof.

Controlled flocculating agents are also known as stabilizing additivesfor solvent-based paint systems. Such systems are generally high inpigment content, i.e. they have a solid content of the order of 30% byweight. Addition of said stabilizing additives leads to a controlledflocculation of the solids, i.e. “bridges” are formed between the singlesuspended pigment particles, causing an “airy” 3-dimensional structureto develop, which fills up the entire volume of the liquid. Paintsobtained through so controlled flocculation of the pigment show markedthixotropy and a less compact specific weight of the flocculatedprecipitate than that resulting from mere pigment sedimentation in theabsence of the flocculating agent. In particular, by choosingappropriate conditions, the flocculated precipitate occupies the entirevolume of the paint, and flooding floating, settling and sagging of thepigment are completely avoided.

The ink composition according to the invention can be applied forprinting methods which use liquid inks. Such liquid printing inks usedmay be for example drop-on-demand inks, gravure inks, helio-flexo inks,screen-printing inks, ink-jet inks etc. Preferably a compositionaccording to the invention may be used for ink-jet printingapplications.

Coating additives to achieve controlled flocculation are known to theskilled in the art and are commercially available. In particular, the“Anti-Terra®” and “Bykumen®” products of BYK Chemie GmbH, Wesel, Germanyare well known stabilizing agents which are recommended for solventbased paint system. They act precisely as controlled flocculating agentswhich stabilize the pigment dispersion by forming a loose 3-dimensionalnetwork between the individual pigment particles. Noteworthy,Anti-Terra®-203 is a solution of an alkyl-ammonium salt of apolycarboxylic acid; Anti-Terra®-204/205 are solutions of apolycarboxylic acid salt of polyamine amides; Anti-Terra®-206 is asolution of an alkylolammonium salt of an unsaturated fatty acid; andBykumen® is a solution of a lower molecular weight unsaturated acidicpolycarboxylic acid polyester. Similar products are also available fromother suppliers (for example EFKA, the 5xxx range of products).

Initially it was tried to “stabilize” an ink-jet ink in a similar way asthe one recommended for stabilizing paints. However, because the solidcontent of an ink-jet ink is only of the order of 3% of the inks totalweight, e.g. about ten times less than the solid content of a paint, theaddition of Anti-Terra® to the suspension of pigments did not result inpreventing pigment settling, as it was first expected, but, in contrary,it was strongly accelerating the pigment settling process.

Surprisingly, we found that the Anti-Terra® flocculated pigment formed asoft sediment, which was easy to redisperse, and which could be printedafter redispersion without any clogging of the printer nozzles. To allevidence, the admixture of such additives to pigmented ink-jet inksacted upon the suspension of solids in a very similar way as in paintsby forming reversible three-dimensional agglomerates. Contrary to whathappens in paints, the three-dimensional agglomerate formed frompigmented ink-jet inks has not a sufficient volume to occupy on one handthe whole space of the liquid; thus, precipitation is observed. On theother hand, surprisingly, the sediment formed in such a way is by farnot as compact as the sediment formed in the absence of saidflocculating additive; the flocculated sediment remains redispersible,and the redispersed liquid remains ink-jet printable.

The concentration of the flocculating agent must be chosen such as toproperly interact with the available particle surface. Usefulconcentrations are in the range of 0.01 to 20% of the inorganic pigmentweight depending on the nature and the mole weight of the flocculent, onthe polarity of the solvent in the fluid, as well as on the availableparticle surface. A too low concentration of flocculating agent willresult in an insufficient agglomeration (the particles are only partlycovered with flocculent); too high concentrations will result in a goodflocculation effect, but with excessive flocculent available which leadsto unwanted secondary effects.

Further, additional compounds like dispersing agents (such as myristicacid, Gantrez AN, Zephrym etc.), defoamers (such as BYK®-022(polyglycols), BYK®-024 (polysiloxane), humectants (such as glycols,glycols ethers, N-methyl-pyrrolidone, etc.), conductivity agent(electrolytes such as LiNO₃, tetrabutyl ammonium chloride, sodiump-toluensulfonate, etc. ), plasticisers (e.g. citroflex, phtalates,etc.), adhesion promoters (silanes, titanium organates, etc.) andbiocides, can be added to the ink-jet ink composition.

EXEMPLARY EMBODIMENTS

A first embodiment of the invention concerns an ink-jet ink comprisingan erbium/ytterbium activated up-converting yttrium oxysulfideluminescent of the formula Y₂O₂S:Er,Yb. The luminescent product has amean particle size of the order of one micrometer. The ink comprisesfurther a polymeric thermoplastic binder (Polyvinylbutyral resin)dissolved in a solvent (Ethanol), a surfactant to support the dispersionof the pigment, a salt to ensure electrical conductivity of the ink, ahumectant to prevent printer nozzle clogging and the controlledflocculating agent Anti-Terra®-204 (or EFKA 5054). Good printability andgood redisperseability were obtained for this embodiment.

A good redisperseability of the pigmented continuous-ink-jet printingink can also be obtained, according to the present invention for anaqueous suspension of the inorganic pigment, applying the same principleof controlled flocculation.

A second embodiment of the invention concerns an ink-jet ink comprisinga down-converting luminescent pigment (a copper-activated zincsulphide), having a mean particle size of 300 nanometers. It alsocomprises a dispersant/binder (polycarboxylic acid sodium salt) andwater as the solvent. The stable suspension could be reversiblyflocculated upon the addition of a polymeric flocculating agent of thesaid types. Alternatively, reversible flocculation could also be inducedby the addition of sodium hydroxyde, FeCl₃ or by the use of aflocculating agent acting on the electrostatic stability of thesuspension (ZetaG). The resulting meta-stable suspension is partiallysedimented (soft sedimentation) over 24 hours but is readily redispersedupon the application of a small mechanical movement. Moreover theprintability and luminescence signal of the prints are not affected bythe addition of the “flocculating” agent.

A good redispersability of the pigmented continuous-ink-jet printing inkcan also be obtained, according to the present invention, by providingthe pigment in two different states; part of the pigment having a softsedimentation behavior (controlled weak flocculation), and part of thepigment having a harder sedimentation behavior. The mixture of the twotypes of suspension results in a fractional sedimentation effect as canbe seen in the third embodiment.

A third embodiment of the invention concerns thus an ink-jet inkformulation comprising an erbium/ytterbium activated up-convertingyttrium oxysulfide luminescent of the formula Y₂O₂S:Er,Yb. Theluminescent product has a mean particle size around one micrometer. Theformulation also comprises a polymeric thermoplastic resin, dissolved ina solvent (Ethanol/Methyl-Ethyl-Ketone, MEK), a surfactant to supportthe dispersion of the pigment, a salt to ensure electrical conductivityof the ink, a humectant to prevent printer nozzle clogging and thecontrolled flocculating agent, Anti-Terra® (or EFKA) as additives.

As described in the first embodiment, a first part of the pigmentsuspension (fluid A) is flocculated by using a polymeric flocculatingagent in the milling process. The obtained suspension (fluid A) is thenadded to a second part of the pigment suspension (fluid B), which isfree of flocculating agent, and mixed thoroughly, to result in aprinting liquid exhibiting a differential sedimentation effect. Theweight ratio of the flocculated suspension (fluid A) to the nonflocculated suspension (fluid B) can vary between 0.4 to 2.5 in theoverall formulation.

Therefore if the hard sedimentation of the non flocculated part of thepigment occurs later in time than the soft sedimentation of theflocculated part of the pigment, the whole sediment can be very easilyredispersed. This is because its bottom consists of “soft sediment”which is easy to redisperse and which takes as well the overlayer ofhard sediment into dispersion. Moreover, the printability of the ink isnot affected by the presence of the partially flocculated pigment; assoon as the ink is stirred and circulated, the flocculated agglomeratesbreak up again into individual particles.

The pigment used as a redispersion help must not necessarily be of thesame type as the luminescent security pigment, as shown in the nextembodiment:

A fourth embodiment of the invention concerns an ink-jet ink, comprisingan erbium/ytterbium activated up-converting yttrium oxysulfideluminescent pigment of the formula Y₂O₂S:Er,Yb, as a first pigment whichhas a mean particle size around one micrometer. Said first pigment isdispersed in a fluid A, comprising a polymeric binder (vinyl chloridecopolymer), a solvent (MEK), a surfactant to disperse the pigment, ahumectant (Dowanol DPM), and a salt to impart electrical conductivity tothe ink. The second pigment is a down-converting luminescent (acopper-activated zinc sulfide), having a mean particle size of 300nanometers, which is dispersed in fluid B. Fluid B also comprises apolymeric binder (vinyl chloride copolymer), a solvent (MEK), asurfactant to disperse the pigment, a humectant (Dowanol DPM), a saltand a controlled flocculating agent.

By mixing fluid A and fluid B, in proportions A:B varying between 0.4and 2.5 of weight ratio a printing liquid is obtained, exhibiting adifferential sedimentation effect, producing a bottom layer of softsediment, covered by a layer of hard sediment. The whole sediment iseasily redispersed. Moreover this printing fluid has both, goodprintability imparted by fluid A, and good redispersion propertiesimparted by fluid B.

The invention will be further outlined by the following formulationexamples, which are intended to be purely illustrative of the inventionand which are in no way limiting its scope:

EXAMPLE 1

This example illustrates the preparation of an embodiment of the presentinvention wherein the total amount of inorganic security pigment wasdispersed in the presence of the controlled flocculating agent,Anti-Terra®, forming a meta-stable suspension ink. Appropriate dosing ofthe controlled flocculating agent in a weight ratio between 0.1 and 10%,preferably of 0.5 to 3%, of the inorganic pigment's weight (depending onthe nature and particle size of the pigment and on the polarity of thesolvent), is used to produce soft sediments which are easily redispersedand whose dispersion is stable enough under stirring and ink circulationto offer a good printability of the ink. At higher weight ratio, theflocculation is too fast, leading to an unstable ink which sedimentsvery quickly and results in a poor printability. At lower weight ratio,the stable suspension settles too slowly, producing a hard sedimentwhich requires high shear rate mixing to redisperse it. The followingingredients were employed in the indicated weight amounts.

Ingredients for the millbase 1 Parts Ethanol 240 Pioloform BL 18 (WackerChemie) 80 Zephrym PD 7000 (ICI) 10 Anti-Terra 204 (Byk Chemie) 0.4Yttrium oxysulfide luminescent Y₂O₂S:Er, Yb 40

The inorganic pigment was first pre-dispersed in the above millbaseusing a high shear rate mixer for about 15 minutes. Remaining pigmentagglomerates in the millbase were then broken up by a bead millingpassage (ZrO₂ beads, 2 mm diameter).

Ingredients for the ink 1 Parts Millbase 1 370 LiNO₃ (Fluka) *** 15Ethanol 595 Dowanol DPM (Dow) 20 *** providing electric conductivity

The ingredients were mixed together to yield the ink-jet ink accordingto the present invention.

EXAMPLE 2

This example illustrates the preparation of another embodiment of thepresent invention wherein the total of the inorganic security pigmentpresent was dispersed in a solvent (water) in the presence of apolycarboxylic sodium salt dispersant, thus forming a stable suspension.The controlled flocculating agent was introduced after the milling stepinto the stable suspension, partially destabilising the ink and favoringits redispersion. The following ingredients were thus employed in theindicated weight amounts.

Ingredients for the Millbase 2 Parts Water 106 Sodium Hydroxide (Fluka)4 Gantrez AN BF 119 (ISP) 8 Copper-activated zinc sulphide pigment 42

The inorganic pigment was first pre-dispersed in the above millbaseusing a high shear rate mixer for about 15 minutes. Remaining pigmentagglomerates in the millbase were subsequently broken up by a beadmilling passage (ZrO₂ beads, 2 mm diameter). The NaOH serves in thiscontext as a neutralizing agent for the binder component.

Ingredients for the ink 2 Parts Millbase 2 160 Water 800 ZETAG 7125(Ciba SC) 4 Dowanol DPM (Dow) 20

The ingredients were mixed together to yield the ink-jet ink 2 accordingto the present invention.

EXAMPLE 3

This example illustrates the preparation of an embodiment of the presentinvention wherein only part of the inorganic security pigment presentwas dispersed in the presence of the controlled flocculating agent,Anti-Terra®-203, to form a meta-stable suspension ink (ink 3A). Theremaining part of the pigment was dispersed in the absence of controlledflocculating agent, forming a stable suspension ink (ink 3B). Finallythe two inks parts were mixed in an appropriate ratio, to produce an inkshowing a fractional sedimentation effect (i.e. a rapidly falling outsoft sediment, covered by a slowly falling out hard sediment), which hasboth, good redispersability and good printability.

Appropriate dosing of the controlled flocculating agent in ink A in aratio between 0.1 and 10%, by weight, of the inorganic pigment producesa soft sediment which is easily redispersible but whose dispersion isstable enough to offer good printability. The weight ratio offlocculating agent to inorganic pigment more preferably lies between 0.5to 3% to provide optimum redispersion properties to the ink of example3. At higher ratio the flocculation is too fast, leading to an unstableink which sediments very quickly and has poor printability. At lowerratio, the stable suspension settles too slowly, producing a hardsediment which requires high shear force mixing to redisperse. Thefollowing ingredients were employed in the indicated amounts.

Parts Ingredients for the millbase 3A Methyl Ethyl Ketone 100 ParaloidDM-55 (Rohm & Haas) 30 Ethanol 120 Pioloform BL 18 (Wacker Chemie) 40Myristic acid (Fluka) 5 Anti-Terra 203 (Byk Chemie) 1 Yttrium oxysulfideluminescent Y₂O₂S:Er, Yb 40 Ingredients for the millbase 3B Methyl EthylKetone 100 Paraloid DM-55 (Rohm & Haas) 30 Ethanol 120 Pioloform BL 18(Wacker Chemie) 50 Myristic acid (Fluka) 10 Yttrium oxysulfideluminescent Y₂O₂S:Er, Yb 40

The inorganic pigments were first pre-dispersed in the above millbasesusing a high shear force mixer for about 15 minutes. Remaining pigmentagglomerates in the millbase were then broken up by a bead millingpassage (ZrO₂ beads, 2 mm diameter).

Parts Ingredients for the ink 3A Millbase 3A 310 Tetrabutylammoniumchloride (Fluka) *** 10 Ethanol 660 Dowanol DPM (Dow) 20 Ingredients forthe ink 3A Millbase 3B 310 Tetrabutylammonium chloride (Fluka) *** 10Ethanol 660 Dowanol DPM (Dow) 20 Ingredients for the ink 3 Ink 3A 500Ink 3B 500 *** providing electric conductivity

The ingredients were mixed together to yield the ink-jet ink accordingto the present invention.

EXAMPLE 4

This example illustrates the preparation of an embodiment of the presentinvention, comprising two inorganic security pigments, wherein only oneof said two inorganic security pigment was dispersed in the presence ofa suitable surfactant and then reversible flocculation was obtained uponthe addition of the conductivity agent (electrostatic destabilisation).The meta-stable suspension ink (ink 4A) was therefore the result of thecombined action of the dispersing surfactant and of the electrostaticdestabilizing conductivity agent. The other of said security pigmentswas dispersed in the absence of any flocculation, forming a stablesuspension ink (ink 4B). Finally to prevent hard settling of ink 4B, thetwo inks were mixed in an appropriate ratio to produce an ink showing afractional sedimentation effect, (i.e. a rapidly falling out softsediment, covered by a slowly falling out hard sediment), which hasboth, good redispersability and good printability.

Appropriate dosage of the surfactant which acts in the presence of thepresent metal salt as a controlled flocculating agent in ink 4A wasgiven by a weight ratio between 0.1 and 10% of the inorganic pigment, tocreate a soft sediment which was easily redispersed but whose dispersionwas stable enough to offer good printability. The more preferred weightratio of flocculating agent to inorganic pigment was found to liebetween 2 to 8%, to provide for optimum redispersion of the ink ofexample 4A. At higher ratio the flocculation is too fast, leading to anunstable ink which sediments very quickly and has a poor printability.At lower ratio, the stable suspension settles too slowly producing ahard sediment which requires high shear force mixing to redisperse. Thefollowing ingredients were employed in the indicated amounts.

Parts Ingredients for the millbase 4A Methyl Ethyl Ketone 250 UCAR VMCAvinylic resin (Dow) 130 Octylphosphonic acid 6 copper-activated zincsulfide pigment 100 Ingredients for the millbase 4B Methyl Ethyl Ketone250 UCAR VMCA vinylic resin (Dow) 100 UCAR VROH vinylic resin (Dow) 30Myristic acid 50 Yttrium oxysulfide luminescent Y₂O₂S:Er, Yb 100

The inorganic pigments were first pre-dispersed in the above millbasesusing a high shear force mixer for about 15 minutes. Remaining pigmentagglomerates in the millbase were then broken up by a bead millingpassage (ZrO2 beads, 2 mm diameter).

Parts Ingredients for the ink 4A Millbase 4A 500 Methyl Ethyl Ketone 470Tetrabutylammonium chloride (Fluka) *** 10 Dowanol DPM (Dow) 20Ingredients for the ink 4B Millbase 4B 470 Methyl Ethyl Ketone 500Tetrabutylammonium chloride (Fluka) *** 10 Dowanol DPM (Dow) 20Ingredients for the ink 4 Ink 4A 500 Ink 4B 500 *** providing electricconductivity

The ingredients were mixed together to yield the ink-jet ink accordingto the present invention.

1. A redispersible pigmented printing ink composition for the continuousink-jet printing process, said ink composition having a viscosity in therange of between 2 and 10 mPas said ink composition comprising at leastone solvent selected from the group consisting of water and organicsolvents of the group of alcohols, ketones and esters, a binder systemin an amount between 0.5% and 20% by weight of the total ink whichcomprises at least one polymer compound selected from the groupconsisting of thermoplastic resins, and at least one inorganicluminescent pigment selected from the group consisting of crystallinecompounds comprising activator-doped zinc sulfides, oxysulfides,vanadates, fluorides, oxides and oxyfluorides of yttrium or therare-earth elements of the periodic table, and mixtures thereof,conductivity control agents, and at least one flocculating agent forminga re-dispersible sediment of said pigment in said composition, whereinsaid flocculating agent is selected from the group consisting ofinorganic salts of aluminum, calcium and iron (III), and of organicpolymers carrying functional groups able to interact with and bind tothe surface of said pigment particles, and wherein said flocculatingagent is present in a concentration range of 0.01 to 20% of theinorganic pigment weight.
 2. A printing ink composition according toclaim 1, further comprising defoaming, and humectant agents.
 3. Aprinting ink composition according to claim 1, further comprising atleast one dispersing agent.
 4. A printing ink composition according toclaim 1, wherein said flocculating agent is an organic flocculatingagent selected from the group consisting of polycarboxylic acids orsalts of a polycarboxylic acid, polyamines or salts of a polyamine,polycarboxylic acid salts of a polyamine amide, polyamides,alkylolammonium salts of an unsaturated fatty acid, polyacrylates,hydrolyzed polyacrylamide, polyimines, (poly)phosphates,(poly)phosphonates, quaternized polyamines, and polyacrylamides ormixtures thereof.
 5. A printing ink composition according to claim 1,wherein said solvent is water, and said thermoplastic resin is selectedfrom the group consisting of water-soluble resins comprisingvinylpyrrolidone polymers and copolymers, water-soluble polyesters,salts of acrylic and maleic anhydride polymers and copolymers,polyvinylalcohol cellulose derivatives, and polyethyleneimides.
 6. Aprinting ink composition according to claim 1, wherein said solvent isan organic solvent selected from the group consisting of alcohols,ketones and esters, and said thermoplastic resin is selected from thegroup consisting of acrylic resins, vinylic resins, polyvinylbutyralresins, cellulose resins, polyurethane resins, polyester resins,polyamine resins, phenolic resins and epoxy resins.
 7. A process ofusing a printing ink composition according to claim 1 for printingsecurity documents or articles, comprising the steps of providing saidprinting ink composition and printing therewith said security documentsor articles by means of a continuous ink-jet printer.
 8. A processaccording to claim 7, wherein said security document is selected fromthe group consisting of banknotes, identity documents, lottery tickets,and warranty labels.
 9. A method for enhancing the redispersibility ofpigment sediments in a pigmented printing ink composition according toclaim 1 for the continuous ink-jet printing process, said inkcomposition having a viscosity in the range of between 2 and 10 mPas,said method comprising the steps of (a) providing a printing inkcomposition comprising at least one solvent selected from the groupconsisting of water and organic solvents of the group of alcohols,ketones and esters, a binder system in an amount between 0.5% and 20% byweight which comprises at least one polymer compound selected from thegroup consisting of thermoplastic resins, at least one inorganicluminescent pigment selected from the group consisting of crystallinecompounds comprising activator-doped zinc sulfides, oxysulfides,vanadates, fluorides, oxides and oxyfluorides of yttrium or therare-earth elements of the periodic table, and mixtures thereof, andconductivity control agents; and (b) adding at least one flocculatingagent forming a redispersible sediment of said pigment in saidcomposition, wherein said flocculating agent is selected from the groupconsisting of inorganic salts of aluminum, calcium and iron (III), andof organic polymers carrying functional groups able to interact with andbind to the surface of said pigment particles, and wherein saidflocculating agent is present in a concentration range of 0.01 to 20% ofthe inorganic pigment weight.
 10. A method of manufacturing a pigmentedprinting ink composition according to claim 1 for the continuous ink-jetprinting process, said ink composition having a viscosity in the rangeof between 2 and 10 mPas, said method comprising the steps of: (a)providing at least one solvent selected from the group consisting ofwater and organic solvents of the group of alcohols, ketones and esters,a binder system in an amount between 0.5% and 20% by weight whichcomprises at least one inorganic luminescent pigment selected from thegroup consisting of crystalline compounds comprising activator-dopedzinc sulfides, oxysulfides, vanadates, fluorides, oxides andoxyfluorides of yttrium or the rare-earth elements of the periodictable, and mixtures thereof, and conductivity control agents; (b)pre-dispersing the pigment in the mixture of step (a); (c) addingfurther solvent to the mixture of step (b); and (d) adding at least oneflocculating agent, either to the mixture of step (a) or to the mixtureof step (c), wherein said flocculating agent forms a re-dispersiblesediment of said pigment in said composition, wherein said flocculatingagent is selected from the group consisting of inorganic salts ofaluminum, calcium and iron (III), and of organic polymers carryingfunctional groups able to interact with and bind to the surface of saidpigment particles, and wherein said flocculating agent is present in aconcentration range of 0.01 to 20% of the inorganic pigment weight. 11.A method according to claim 10, wherein in step a) furthermoredispersing, defoaming, and humectant agents are provided.